GDP dissociation inhibitor (GDI) is a regulator of small ras-like GTPases of the rab subfamily. The rab GTPases control steps in vesicular transport, involving both secretion and endocytosis. Moreover, in the cell-division mutant of Drosophila, quartet, rab GDI is shifted in isoelectric point, suggesting that GDI may play a role in governing the onset and progression of cell division.